This invention relates to electronic musical instruments and, more particularly, to a tone signal generating apparatus using a digital circuit for electronic musical instruments or the like.
The use of a digital circuit for an electronic musical instrument has various advantages such as the possibility of simultaneously generating a plurality of tones through a time division basis process, the possibility of accurately setting tone frequencies and the possibility of simplifying the circuit construction by using analog circuitry.
In one method of digitally obtaining tone signals, an ROM with tone waveform data preliminarily written therein is used. To obtain a tone signal, addresses of the ROM are designated by the output of an address counter according to operated keys, and tone waveform data corresponding to the given musical notes are read out from the ROM. In this system, however, the ROM address to be designated is fixed irrespective of the tone frequency, that is, the tone waveform is sampled at a fixed point. Particularly, in case of a tone waveform in a high tone range, folding distortion is liable to occur due to sampling theory. A so-called jitter also can be generated when the same address is designated a plurality of times in succession. In order to solve these problems it is necessary to use a ROM having a very large memory capacity. It is conceivable to vary the address step interval for varying the sampling point of the waveform. To do so, however, has a drawback in that the tone color is varied slightly with each musical note.